1. FIELD OF THE INVENTION
This invention relates to a chain for a bicycle, and more particularly, to a chain for a bicycle having an exterior gear shifting assembly provided with a front multi - stage sprocket unit to be shifted by a front derailleur, and a rear multi - stage sprocket unit to be shifted by a rear derailleur.
2. DESCRIPTION OF THE INVENTION
In a conventional exterior gear shifting assembly, a chain set around a sprocket of a multi - stage sprocket unit is pushed laterally and displaced by a derailleur fitted immediately before the sprocket. The chain catches onto the next sprocket at the displaced portion, changing the speed. In order for the chain to shift from a large sprocket to a small one, it slips down of itself and fits into the small sprocket due to tension produced by a rear derailleur, providing that it is pushed toward the small sprocket upstream in the large sprocket running direction, and is disengaged with the larger sprocket.
It is difficult, on the contrary, for the chain to shift from the small sprocket to the large one. If the chain is pushed toward the large sprocket upstream is the small sprocket running direction, it will be merely released from the small sprocket but hardly shift to the large one. It is necessary to push the chain further toward the large sprocket until link plate edges of the chain catch onto teeth of the large sprocket. For example, Japanese Utility Model laid open to public inspection, No. JIKKOUSHO 62 - 29712, discloses the art of modifying the form of a chain to avoid the problem noted above. The disclosed chain has a projection that swells outwardly on the edge of each central portion of the right and left outer link plates, and an outwardly spread recess to lead a tooth of a sprocket. For shifting, the projection catches onto the tooth of the sprocket and then the tooth slips into the outwardly spread recess.
Generally, in an exterior gear shifting assembly, a front multi - stage, sprocket unit has a small number of stages such as two or three stages, while a rear multi - stage sprocket unit had a large number of stages, five or more, for example. And, there is produced a smaller space between the small and large sprockets in the rear multi - stage sprocket unit than in the front multi - stage sprocket unit. This suggests, therefore, that it is preferable for the chain to work on the front - multi sprocket differently than on the rear multi - sprocket unit.
As mentioned above, each edge of the right and left outer link plates of the chain swells outwardly. The projection catches on a tooth of a sprocket to change the speed, and the chain readily shifts from the small sprocket to the large one in the front multi - stage sprocket unit. When it shifts from the small sprocket to the large one in the rear multi - stage sprocket unit, however, the projection of the outer link plate of the chain comes in contact with the large sprocket on the side because the space between the small and large sprockets is narrow. Although the chain is pushed laterally and displaced toward the large sprocket by the derailleur each edge of the projections can not catch onto a tooth of the large sprocket because the sprocket contacts with the projection on the side. Thus, this arrangement is not free from trouble; it fails to shift up the chain.
Recent exterior shifting assemblies for bicycles tend to have a larger number of stages and an increased ratio of speed changes, so the number of stages in the rear multi - stage sprocket unit inevitably increases. Therefore, the space between sprockets will be narrower, and then it may become more difficult for the chain as described earlier to shift, because it has projections on the right and left outer link plates.